What was done
The authors analyzed air and seawater temperature data obtained over the past century at sites of primary tide gauges of the U.S. Coast and Geodetic Survey, calculating trends for the 14 longest records.

What was learned
A mean century-long seawater warming of 0.74°C was derived for the 14 sites.  The result for Boston (a 100-year warming of 3.6°C), however, seemed anomalous in the extreme, being almost seven times greater than the mean of the other 13 stations (0.52°C).  Hence, it should probably be rejected.  Its veracity is also called into question by the fact that Boston was the only site where seawater temperature increased faster than air temperature.

What it means
First of all, a mean century-scale warming of 0.52°C does not seem extraordinary for the 20th century, when the planet experienced the bulk of its recovery from the global chill of the Little Ice Age.  Nevertheless, the authors note that "each site has experienced significant population growth in the last 100 years, and ... with the increase in maritime traffic and discharge of wastewater one would expect water temperatures to rise," which implies that some unspecified portion of the half-degree C warming is due to a maritime analogue of the urban heat island, making the true century-scale non-urban-influenced increase in coastal seawater temperature probably significantly less than 0.5°C.

The authors also note that "on the time scales investigated herein, one would expect the water temperatures to equilibrate to the air."  Hence, they state that the fact that the air temperature trends at the tide gauge sites are "much larger" than the seawater temperature trends "is unanticipated."  And what would cause the local air temperature trends to be higher than anticipated?  The first thing that comes to our minds is the good ol' reliable urban heat island effect.

What we seem to have, therefore, are urban-inflated air temperature trends that are larger than urban-inflated coastal seawater temperature trends.  So if we want to know something about non-urban-influenced air temperature trends along the east and west coasts of the United States, this paper would seem to suggest that the true 100-year warming is something significantly less than something else that is significantly less than 0.52°C.  Which prompts us to wonder, relative to the true background air temperature trend, How low can it go?